Targeting the mevalonate pathway in multiple myeloma

Multiple myeloma is an incurable plasma cell malignancy. Although novel agents have improved overall survival in the last decade, multidrug resistance eventually still occurs in all patients. New treatment strategies are therefore needed. Statins are HMG-CoA reductase inhibitors that are well known as treatment for hypercholesterolemia. In addition, in vitro studies show that statins also effectively induce apoptosis and inhibit proliferation in myeloma plasma cells by inhibition of geranylgeranylation. This was the rationale for a phase I dose-escalating study combining high-dose simvastatin with chemotherapy in extensively pretreated lymphoma and myeloma patients to prove feasibility of this regimen (chapter 2). The maximum-tolerated dose was simvastatin 15mg/kg/day for 7 days, followed by chemotherapy. Dose-limiting toxicities were neutropenic sepsis and gastrointestinal side effects. Other side effects grade I-II consisted of fatigue, gastrointestinal complaints, and neutropenic fever. In a phase II study, we evaluated the efficacy of high-dose simvastatin (15 mg/kg/day) followed by VAD chemotherapy in patients with refractory or relapsed multiple myeloma (chapter 3). Although the feasibility of this regimen was confirmed in this trial with only mild side effects observed, we found that after treatment of 12 patients, only one patient achieved a partial response. According to our predefined criteria this was insufficient to continue the study. To improve the efficacy of statins in multiple myeloma, we searched for other agents showing synergy when combined with simvastatin. We showed that co-exposure to simvastatin and lenalidomide resulted in a synergistic reduction of cell viability in myeloma cells (chapter 4). This effect was due to induction of apoptosis and inhibition of proliferation. The combination augmented induction of caspase-8 cleavage and enhanced down-regulation of pStat3. Mevalonate and GGOH abrogated the synergy between lenalidomide and simvastatin. These data provide a rationale for the clinical evaluation of lenalidomide and simvastatin in patients with myeloma. Although the antimyeloma effect of statins was shown to act through inhibition of the mevalonate pathway, its downstream targets remain elusive. Analysis of alterations in gene expression can help to identify targets of statins in myeloma. The myeloma plasma cell line XG-1 was treated with simvastatin alone and with combinations of simvastatin and agents that rescue the myeloma cell from simvastatin-induced cell kill (mevalonate, GGOH and FOH)(chapter 5). Subsequently, relative gene expression pro?les were assessed. Simvastatin treatment resulted in a two-fold differential expression of 535 genes compared with control cells. The ten most upregulated and downregulated genes by simvastatin included RhoB (up), EGR-1, CCR2, cdc25a and cdk6 (down). Analysis of differentially expressed genes using Ingenuity pathway software showed that most of the genes are related to cancer and are associated with cell death and growth and proliferation, including cell cycle. The most affected canonical pathway was the G1/S checkpoint regulation. In chapter 6, we present a myeloma mouse model suitable for preclinical testing of novel strategies, using bioluminescence. To conclude, although statins have very effective anti-myeloma efficacy in vitro, current strategies combining high-dose statins with chemotherapy lacks satisfactory efficacy. New strategies need to be considered, to make use of its in vitro efficacy.